J. Sci. Food Agric. 1982, 33, 1324-1326

The Effect of Heating on the Haemagglutinating Activity and Nutritional Properties of Bean (Phaseolus vuZgaris) Seeds

George Grant, Linda J. More, Norma H. McKenzie and Arpad Pusztai

Roweft Research Insiifuie, Bucksbum, Aberdeen AB2 9SB

(Manuscripi received 26 January 1982)

The toxic lectins present in red, white and black kidney beans (Phaseolus vulgaris) are sensitive to heat treatment and the efficiency of that treatment is greatly improved by pre-soaking of the seeds. Heating of pre-soaked seeds at all temperatures above 75°C caused a continuous reduction in both their haemagglutinating activity and toxicity. However, the only safe method of eliminating toxicity was to heat the fully hydrated seeds to 100°C for a minimum of 10 min.

1. Introduction

The consumption of raw kidney beans (Phaseolus vulgaris) has been shown to cause gastroenteritis, nausea and diarrhoea in humans1, 2 , 3 and rapid weight loss and death in rats4 and quail.5

Seeds have long been known as a source of protein which agglutinate erythrocytes of various origins.6 Some of these haemagglutinins (Iectins) were shown to be toxic for mice when injected intraperitoneally and were also thought to contribute to the poor nutritive quality of beans.7 It has now been established that the toxic factor in kidney bean is identical with its constituent lectins and that the level of toxicity can be directly related to the lectin content and hence the haemagglutination a~ t iv i ty .~ Thus it is now well established that the high content of haemagglutinating lectins is responsible for the toxicity to humans 192 rats4 and quaiL5 Although some of this lectin is reported to be leached out during soaking of the beans,l for safe removal of the toxic effects an appropriate heat treatment of the seeds is necessary. The recent suggestion that cooking of beans at 80°C actually increases the amount of toxin five-folds highlights the need to understand fully the pro- perties and behaviour of these lectins during heat-treament as well as the need to relate changes in haemagglutinating activity to potential changes in nutritional performance.

2. Materials and methods 2.1. Beans

The variety of white kidney beans (Processor) was obtained from Hurst Gunson Cooper Taber Ltd, Witham, Essex, the red kidney bean from Aberdeen Grain and Herb Store and the black kidney bean from Nature’s Larder, Aberdeen. Lactic unextracted casein was supplied by Glaxo Laboratories Ltd, Greenford, Middlesex.

2.2 Diets Diets were prepared as described previouslys and contained a total of 100g protein kg-1 of

casein or Processor bean. The protein sources were fed without supplementation with any indi- vidual amino acids.

2.3. Sample preparation and haemagglutination Samples were prepared by soaking dry beans for 16 h and then heating them in water (300 g wet weight litre-l) at temperatures up to 100°C for various lengths of time. One sample of beans was

0022-5142/82/120&1324 902.00 0 1982 Society of Chemical Industry

1324

Effect of heating on bean seeds 1325

heated, in water, at 100°C without pre-soaking. Both soaking and cooking water extracts were discarded. The heat-treated beans were freeze-dried and ground in a Moulinex Junior coffee grinder.

The flour of both the untreated and heat-treated bean samples was suspended in 0.04 M Na borate (pH 8.0) and the suspension tested for haemagglutinating activity by the serial dilution procedure described previouslylo except that the rabbit erythrocytes were diluted 20 fold and the initial sample concentration was 5 % (w/v). One unit of haemagglutination activity (H.U.) was defined as the amount of material ml-l in the last dilution giving 50% agglutination. Activities were expressed as the weight of material (pg) containing one H.U.

2.4. Nutritional evaluation Net Protein Utilisation (NPU) was determined by a methodY based on that described by Miller and Bender.ll

2.5. Chemical analyses Diets, carcass and ground faeces samples were analysed for moisture content and total nitrogen.12

3. Results and discussion Although there was no detectable change in the haemagglutinating activity of presoaked samples heated at 50, 65 and 7 0 T , this activity was clearly sensitive to heat-treatment at higher temperatures (Figure 1) . Thus when samples were heated at 75, 80 and 85", the activity was progressively reduced

Figure 1. The effect of heat treatment on the haemagglutin- ation activity of the white kidney bean (Plraseolus vulgaris var. Processor). The pre- soaked samples were heated at various temperatures. 0- 0 , 5 0 , 65or70"C; 0-0.75"C; A-A, 80°C; A-A, 85°C; C3-0, 90°C; W-W., 9 5 T , Y-V, 100°C; v - v , unhydrated sample heated at 100°C.

0- F c t

0

yl c 0

n c 0

Heating time ( n )

although not completely eliminated. Activity was abolished when samples were heated for 4 h at 90°C, 45 rnin at 95°C or 4-10 min at l00%C. Heat treatment of all three kidney bean samples gave identical results. Thus, contrary to that reported,* no increase in haemagglutination activity was found at any temperature.

There were no detectable differences found in the haemagglutination activity of extracts prepared from dry or fully hydrated seeds when the beans were not heat treated. However, the importance of presoaking was shown in that, with presoaked samples the haemagglutination activity was eliminated by heat-treatment at 100°C for 4-10 rnin whilst it required 90 min at 100°C to abolish the activity of a dry bean sample.

Selected samples, presoaked for 16 h, were also tested for nutritional performance (Figure 2). Rats fed on beans heated at 100°C for 10 min gained 20 g in weight and gave an NPU value of 45 while the unheated sample caused a continuous weight loss far greater than that of the non-protein control and the experiment had to be terminated after two days when some of the rats died. However, rats fed on a bean sample heated at 80°C for 3 h, although surviving the 10 day experimental period, lost more weight and body N than did the non-protein control and had a negative NPU value. This

1326 G. Grant et nf.

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Figure 2. The growth curves for groups of four rats fed treated samples of white kidney bean (Phaseolus vulgaris, var. Processor). The samples were: presoaked but unheated 0-0; pre- soaked and heated for 3 hr at 80°C A- A ; and presoaked and heated for 10min at 100°C A- A. The reference diets were the non-protein

control 0- and casein 0- 0.

290 -

Time (days )

behaviour appeared to reflect the decrease, though not the abolition, of the haemagglutination activity of beans heated at 80°C for 3 h. These beans were thus measurably less toxic than the original raw beans.

These results also suggested that heat treatments at moderate temperatures, including those obtained in 'Slow cookers' which operate between 6O-85"C,l3 even over a long period, are not sufficient to abolish toxicity. The only safe method of eliminating the toxicity was by treating the fully hydrated seeds at 100°C for a minimum for 10 min.

Finally, the results suggest, as did earlier ones,14 that the toxicity and the haemagglutinating activity of beans are related and, furthermore, measurement of haemagglutinating activity is a safe method for checking the efficiency of heat treatment of toxic seeds.

Acknowledgements This work was supported in part, by a grant from the Scottish Home and Health Department.

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